1. Field of the Invention
The present invention relates to mechanical analysis of a material used to make a medical device or portions thereof intended for implantation within a body; more particularly, this invention relates to mechanical analysis of a polymeric material represented by a hollow tube having dimensions approximating the dimensions of a medical device, such as a stent.
2. Background of the Invention
A Dynamic Mechanical Analyzer (DMA) is a precision instrument designed to measure the viscoelastic properties of a material, such as a polymer material in a dry or wet stage. A DMA may be used to measure changes in a sample material resulting in changes in temperature and/or external forces. Applied external forces may be represented by enforced displacements on the sample, in which case material properties are determined from a measured reaction force. The external forces may be time-varying, e.g., sinusoidal. Prior to testing, a sample of the material is mounted in a clamp, one part of which is stationary and the other part is moving and connected to a motor drive.
The sample can be in a bulk solid, film, fiber, gel or viscous form depending on the fixture used. The motor drives the sample to a selected strain or amplitude. As the sample undergoes deformation, a linear variable differential transformer mounted on a driving arm or rod measures such quantities as a static or time-varying strain amplitude as feedback control to the motor. Interchangeable fixtures are used to measures quantities such as an elastic modulus, toughness, damping, stress relaxation, creep, and softening points. See e.g., Introduction to Dynamic Mechanical Analysis (DMA)—A beginner's Guide, PerkinElmer® Inc.
One quantity of importance in analyzing material used to make implantable medical devices is the glass transition temperature, Tg. The “glass transition temperature,” Tg, is the temperature at which the amorphous domains of a polymer change from a brittle vitreous state to a solid deformable or ductile state at atmospheric pressure. In other words, the Tg corresponds to the temperature where the onset of segmental motion in the chains of the polymer occurs. When an amorphous or semicrystalline polymer is exposed to an increasing temperature, the coefficient of expansion and the heat capacity of the polymer both increase as the temperature is raised, indicating increased molecular motion. As the temperature is raised the actual molecular volume in the sample remains constant, and so a higher coefficient of expansion points to an increase in free volume associated with the system and therefore increased freedom for the molecules to move. The increasing heat capacity corresponds to an increase in heat dissipation through movement. Tg of a given polymer can be dependent on the heating rate and can be influenced by the thermal history of the polymer. Furthermore, the chemical structure of the polymer heavily influences the glass transition by affecting mobility.
The sample and the fixture restraining the sample is enclosed within a thermal isolation chamber which can heat the sample and the fixtures to temperatures above normal ambient temperatures or cool the sample and the fixtures to temperatures below normal ambient temperatures. The temperature is generally varied dynamically, e.g., at a constant heating or cooling rate. The stiffness and damping of a sample may be calculated as a function of temperature from force, displacement and phase data, using well-known mathematical relationships which separate the applied load into the components due to movement of the mechanical system and the components due to deformation of the sample. The phase relationship between the force applied to the sample and the resultant displacement allows the sample deformation force component to be further divided into an elastic component and a viscous component. The elastic and viscous components are used to determine the elastic modulus and damping through the use of model equations for the particular sample geometry and deformation mode. These equations are well-known in the field, e.g., Theory of Elasticity, S. P. Timoshenko and J. N. Goodier, McGraw-Hill (3rd ed. 1970). Currently, there are two classes of fixtures for DMA—tensioning and non-tensioning. Tensioning fixtures include the 3-point bend, tension/film, tension/fiber, compression, compression and penetration fixtures, while non-tension fixtures include single/dual cantilever and shear sandwich fixtures.
As mentioned above, existing fixtures for restraining movement of a sample in a DMA are intended for analysis of material in bulk solid, film, fiber, gel or viscous form. Unfortunately, there exists no ability to test a hollow cylindrical tube, in particular, a thin walled tube having dimensions corresponding to a tube that is formed into a stent. A fixture suited for a film or fiber, for example, cannot restrain a hollow tube in a wet or dry environment, as needed, because either the sample cannot be adequately held by a clamp, the clamp induces unwanted preloads into the sample, such as torsion, or collapses the tube walls when the clamp is tightened down on the ends of the tube in order to hold it in place. Other problems with existing fixtures are they are difficult to assemble or modify to accommodate the special needs of a hollow tube.